nfs-kernel-server: do not export /mnt by default

nfs-kernel-server: do not export /mnt by default

Currently, the nfs-kernel-server package exports /mnt by default after
it is installed. This is not a good default behavior, as it may expose
sensitive data to the network if a user mounts something on /mnt. This
commit commented out the line that exports /mnt, so the user has to
enable it explicitly.

Signed-off-by: Yangyu Chen <cyy@cyyself.name>